According to a recent study, motor vehicle crashes are up 6 percent in four states that have legalized recreational marijuana compared with four neighboring states where the drug is restricted or illegal.

The study was conducted by the Insurance Institute for Highway Safety (IIHS) and Highway Loss Data Institute (HLDI), which say the crash data suggest that as more states legalize recreational marijuana, more effort will be needed to determine how best to prevent impaired driving crashes.

“What we’re seeing is a definite increase in crash risk that is associated with the legalized recreational use of marijuana,” says David Harkey, president of IIHS and HLDI.